In the design of a disc brake caliper for an automotive application, it is important to consider the effect the caliper has on the pedal feel that the driver experiences. For the best sense of control, and minimum reaction times, the pedal should have a firm feel, with a minimum of free travel before engagement. The structural stiffness of the caliper is the primary contributor to the firmness of the pedal, and should be made as high as possible within a reasonable mass. A limitation to the structural efficiency of current caliper designs is the need to machine the cylinder bore. This operation requires a large access notch in the outboard side of the caliper, causing a structural discontinuity.
Free travel results largely from the need to retract the brake pads slightly from the surface of the brake rotor to minimize the drag of the brakes when they are not being applied. A small but consistent value for the amount of retraction is desired. The retraction function is typically performed in disc brake calipers by the elastomeric seal between the piston and the cylinder bore. Most current designs use an o-ring of square cross-section located in a groove machined into the inside wall of the cylinder bore. The extension of the piston due to the application of hydraulic pressure, causes the seal to be deformed in shear by the friction forces at the surface of the piston. When the hydraulic pressure is released, the piston is drawn back slightly by the elastic stresses stored in the seal. One disadvantage of this approach is that the retraction force produced by the seal must overcome the somewhat variable friction forces between the cylinder bore and the close fitting piston. Variability is created by normal manufacturing tolerances for the piston and cylinder bore, cocking of the piston due to caliper deflection and uneven brake pad wear, corrosion or dirt between the piston and bore, thermal expansion of the piston, and the shape and dimensions of the o-ring groove. This variability can be controlled to some degree by tightening the manufacturing tolerances on the various components, but this adds cost.